Voice and fax transmissions are currently sent using two types of communication networks: the classical, circuit-oriented telephone network used primarily for transmitting voice, and the packet-oriented data network, which is set up for transmitting data. The circuit-oriented telephone network is referred to as a circuit network for short below, and the packet-oriented data network is referred to as a packet network.
The classical circuit networks are not just used for interchanging information between the parties to a call. Instead, information relating to the control of the circuit network itself is also interchanged. This information is not necessarily passed to the parties to a call on the terminals, but rather is evaluated by the network nodes or terminals. The interchange of this information is referred to as signaling. Since defined communication paths are generally reserved for the signaling in the circuit networks, signaling networks are also referred to. In principle, the signaling for a circuit network can also take place over a packet network.
In addition, there is the tendency to also use packet networks to transmit information which are typically routed via the classical circuit network. Since the corresponding information is routed not just via the classical circuit network, but also via packet networks, heterogeneous networks are referred to in this context.
Compressive codecs cannot readily be used for fax transmissions over a packet network, since the use of compressive codecs does not ensure error-free transmission of the fax tones. Although it is possible, in principle, to use a non-compressive codec for transmitting fax transmissions over packet networks, it is necessary to ensure that pauses between the signal tones are not removed from the data stream by the codec. It is thus possible, for example, to use the non-compressive codec from G.711 to set up a fax transmission over a packet network, particularly an Internet Protocol network. However, such a fax transmission requires a bandwidth of 64 kb/sec, even though the fax transmission itself has a data rate of only 14 kb/sec. For this reason the T.38 standard has been created for fax transmissions over a packet network on the basis of the Internet Protocol.
Following the end of a fax transmission, it is occasionally necessary to switch back to a voice transmission which preceded the fax transmission. This has not been possible to date for fax transmissions over a packet network. The subscribers have had to end the fax transmission and set up the desired voice transmission again.